diff --git a/scripts/gulpfiles/build_tasks.js b/scripts/gulpfiles/build_tasks.js
index 8b647b356..5b9213a76 100644
--- a/scripts/gulpfiles/build_tasks.js
+++ b/scripts/gulpfiles/build_tasks.js
@@ -173,7 +173,6 @@ function stripApacheLicense() {
  * https://github.com/google/closure-compiler/blob/master/src/com/google/javascript/jscomp/DiagnosticGroups.java#L113
  */
 var JSCOMP_ERROR = [
-  'accessControls',
   'checkPrototypalTypes',
   'checkRegExp',
   'checkTypes',
@@ -216,7 +215,19 @@ var JSCOMP_ERROR = [
   'unusedPrivateMembers',
   'uselessCode',
   'untranspilableFeatures',
-  'visibility'
+  /* In order to transition to ES modules, modules will need to import one
+   * another by relative paths. This means that the existing practice of moving
+   * all source files into the same directory for compilation (see docs for
+   * flattenCorePaths) would break imports. Not flattening files in this way
+   * breaks our usage of @package however; files were flattened so that all
+   * Blockly source files are in the same directory and can use @package to mark
+   * methods that are only allowed for use by Blockly, while still allowing
+   * access between e.g. core/events/* and core/utils/*. For now, we're
+   * downgrading access control violations (including @private) to warnings.
+   * Once ES module migration is complete, they will be re-enabled and an
+   * alternative to @package will be established.
+   */
+  // 'visibility'
 ];
 
 /**
@@ -537,10 +548,10 @@ function buildCompiled() {
   return gulp.src(chunkOptions.js, {base: './'})
       .pipe(stripApacheLicense())
       .pipe(gulp.sourcemaps.init())
-      .pipe(gulp.rename(flattenCorePaths))
+      // .pipe(gulp.rename(flattenCorePaths))
       .pipe(compile(options))
       .pipe(gulp.rename({suffix: COMPILED_SUFFIX}))
-      .pipe(gulp.sourcemaps.mapSources(unflattenCorePaths))
+      // .pipe(gulp.sourcemaps.mapSources(unflattenCorePaths))
       .pipe(
           gulp.sourcemaps.write('.', {includeContent: false, sourceRoot: './'}))
       .pipe(gulp.dest(BUILD_DIR));
@@ -568,13 +579,14 @@ function buildAdvancedCompilationTest() {
     compilation_level: 'ADVANCED_OPTIMIZATIONS',
     entry_point: './tests/compile/main.js',
     js_output_file: 'main_compressed.js',
+    jscomp_warning: ['visibility'],
   };
   return gulp.src(srcs, {base: './'})
       .pipe(stripApacheLicense())
       .pipe(gulp.sourcemaps.init())
-      .pipe(gulp.rename(flattenCorePaths))
+      // .pipe(gulp.rename(flattenCorePaths))
       .pipe(compile(options))
-      .pipe(gulp.sourcemaps.mapSources(unflattenCorePaths))
+      // .pipe(gulp.sourcemaps.mapSources(unflattenCorePaths))
       .pipe(gulp.sourcemaps.write(
           '.', {includeContent: false, sourceRoot: '../../'}))
       .pipe(gulp.dest('./tests/compile/'));
diff --git a/scripts/gulpfiles/chunks.json b/scripts/gulpfiles/chunks.json
index c034a0d89..46b36a9c6 100644
--- a/scripts/gulpfiles/chunks.json
+++ b/scripts/gulpfiles/chunks.json
@@ -64,23 +64,18 @@
     "./core/utils/useragent.js",
     "./core/utils/svg.js",
     "./core/utils/dom.js",
-    "./core/utils/idgenerator.js",
     "./core/connection_checker.js",
-    "./core/toolbox/separator.js",
-    "./core/toolbox/toolbox_item.js",
-    "./core/interfaces/i_selectable_toolbox_item.js",
-    "./core/interfaces/i_collapsible_toolbox_item.js",
-    "./core/toolbox/category.js",
-    "./core/serialization/exceptions.js",
-    "./core/interfaces/i_serializer.js",
-    "./core/serialization/registry.js",
-    "./core/serialization/priorities.js",
-    "./core/serialization/blocks.js",
-    "./core/utils/toolbox.js",
+    "./core/keyboard_nav/ast_node.js",
+    "./core/keyboard_nav/cursor.js",
+    "./core/registry.js",
     "./core/utils/math.js",
+    "./core/utils/idgenerator.js",
     "./core/utils/array.js",
     "./core/workspace.js",
+    "./core/utils/object.js",
     "./core/events/events_block_delete.js",
+    "./core/toolbox/separator.js",
+    "./core/toolbox/toolbox_item.js",
     "./core/keyboard_nav/basic_cursor.js",
     "./core/keyboard_nav/tab_navigate_cursor.js",
     "./core/warning.js",
@@ -121,7 +116,6 @@
     "./core/options.js",
     "./core/interfaces/i_bounded_element.js",
     "./core/grid.js",
-    "./core/css.js",
     "./core/flyout_button.js",
     "./core/contextmenu_registry.js",
     "./core/theme/classic.js",
@@ -134,7 +128,6 @@
     "./core/renderers/zelos/path_object.js",
     "./core/renderers/zelos/drawer.js",
     "./core/renderers/zelos/renderer.js",
-    "./core/utils/aria.js",
     "./core/field_textinput.js",
     "./core/field_image.js",
     "./core/renderers/zelos/constants.js",
@@ -162,11 +155,8 @@
     "./core/renderers/measurables/input_row.js",
     "./core/renderers/measurables/inline_input.js",
     "./core/scrollbar.js",
-    "./core/interfaces/i_toolbox_item.js",
-    "./core/interfaces/i_toolbox.js",
     "./core/utils/metrics.js",
     "./core/interfaces/i_metrics_manager.js",
-    "./core/interfaces/i_flyout.js",
     "./core/metrics_manager.js",
     "./core/interfaces/i_deletable.js",
     "./core/interfaces/i_draggable.js",
@@ -196,15 +186,14 @@
     "./core/events/events_var_delete.js",
     "./core/variable_map.js",
     "./core/names.js",
+    "./core/events/events_block_base.js",
+    "./core/events/events_block_change.js",
     "./core/events/events_ui_base.js",
     "./core/events/events_marker_move.js",
     "./core/renderers/common/marker_svg.js",
     "./core/keyboard_nav/marker.js",
-    "./core/keyboard_nav/ast_node.js",
-    "./core/keyboard_nav/cursor.js",
     "./core/marker_manager.js",
     "./core/field_label.js",
-    "./core/input_types.js",
     "./core/interfaces/i_registrable_field.js",
     "./core/field_registry.js",
     "./core/input.js",
@@ -227,10 +216,8 @@
     "./core/procedures.js",
     "./core/workspace_svg.js",
     "./core/utils/rect.js",
-    "./core/utils/size.js",
     "./core/utils/coordinate.js",
     "./core/utils/style.js",
-    "./core/utils/deprecation.js",
     "./core/utils/svg_math.js",
     "./core/bubble_dragger.js",
     "./core/block_animations.js",
@@ -239,9 +226,23 @@
     "./core/browser_events.js",
     "./core/tooltip.js",
     "./core/block_svg.js",
-    "./core/events/events_block_base.js",
-    "./core/events/events_block_change.js",
-    "./core/utils/xml.js",
+    "./core/interfaces/i_flyout.js",
+    "./core/interfaces/i_toolbox.js",
+    "./core/interfaces/i_toolbox_item.js",
+    "./core/interfaces/i_selectable_toolbox_item.js",
+    "./core/interfaces/i_collapsible_toolbox_item.js",
+    "./core/utils/aria.js",
+    "./core/utils/deprecation.js",
+    "./core/css.js",
+    "./core/toolbox/category.js",
+    "./core/input_types.js",
+    "./core/utils/size.js",
+    "./core/serialization/exceptions.js",
+    "./core/interfaces/i_serializer.js",
+    "./core/serialization/registry.js",
+    "./core/serialization/priorities.js",
+    "./core/serialization/blocks.js",
+    "./core/utils/toolbox.js",
     "./core/connection_type.js",
     "./core/internal_constants.js",
     "./core/mutator.js",
@@ -251,14 +252,13 @@
     "./core/extensions.js",
     "./core/block.js",
     "./core/utils/string.js",
-    "./core/utils/object.js",
     "./core/dialog.js",
+    "./core/utils/xml.js",
     "./core/events/events_var_base.js",
     "./core/events/events_var_create.js",
     "./core/variable_model.js",
     "./core/variables.js",
     "./core/events/events_abstract.js",
-    "./core/registry.js",
     "./core/events/utils.js",
     "./core/xml.js",
     "./core/connection.js",